Over the years, a variety of implantable electrochemical sensors have been developed for detecting and/or quantifying specific agents or compositions in a patient's blood. For instance, glucose sensors have been developed for use in obtaining an indication of blood glucose levels in a diabetic patient. Such readings can be especially useful in monitoring and/or adjusting a treatment regimen which typically includes the regular administration of insulin to the patient. Thus, blood glucose readings are particularly useful in improving medical therapies with semi-automated medication infusion pumps of the external type, as generally described in U.S. Pat. Nos. 4,562,751; 4,678,408; and 4,685,903; or automated implantable medication infusion pumps, as generally described in U.S. Pat. No. 4,573,994, which are herein incorporated by reference.
Generally, small and flexible electrochemical sensors can be used to obtain periodic readings over an extended period of time. In one form, flexible subcutaneous sensors are constructed in accordance with thin film mask techniques in which an elongated sensor includes thin film conductive elements encased between flexible insulative layers of polyimide sheets or similar material. Such thin film sensors typically include a plurality of exposed electrodes at one end for subcutaneous placement with a user's blood, or the like, and a corresponding exposed plurality of conductive contacts at another end for convenient external electrical connection with a suitable monitoring device. Typical thin film sensors are described in commonly assigned U.S. Pat. Nos. 5,390,671; 5,391,250; 5,482,473; and 5,586,553 which are incorporated by reference herein. See also U.S. Pat. No. 5,299,571.
Drawbacks to conventional implantable sensors arise from the initial subcutaneous insertion and from the extended presence of the sensor at the subcutaneous insertion site. For example, the area surrounding the implantable sensor may swell or fill with fluid that impedes the ability of the implantable sensor to provide accurate results. This represents a potential health hazard, since less accurate information could lead to erroneous dosing of medication or the like. Another drawback that results from a sensor being inserted for extended periods of time is that it is more prone to infection, which is a health hazard and can also result in fluid buildup and inaccurate readings. To overcome these drawbacks of inaccurate readings and the possibility of infection, the implantable sensor is removed from the subcutaneous site and a new implantable sensor is inserted at a different subcutaneous insertion site. While this solution does provide more accurate readings or reduces the occurrence of infections, it is expensive due to the increased number of implantable sensors needed, and is painful for the user who must insert implantable sensors in more locations and on a more frequent basis.